Manual Reference Pages - socat (1)

CONTENTS

Socat is a command line based utility that establishes two bidirectional byte
streams and transfers data between them. Because the streams can be constructed
from a large set of different types of data sinks and sources
(see address types), and because lots of
address options may be applied to the streams, socat can
be used for many different purposes.
It might be one of the tools that one has already needed'.

Filan is a utility that prints information about its active file
descriptors to stdout. It has been written for debugging socat, but might be
useful for other purposes too.

Procan is a utility that prints information about process parameters to
stdout. It has been written to better understand
some UNIX process properties and for debugging socat, but might be
useful for other purposes too.

The life cycle of a socat instance typically consists of four phases.

In the init phase, the command line options are parsed and logging is
initialized.

During the open phase, socat opens the first address and afterwards the
second address. These steps are usually blocking; thus, for complex address types like socks,
connection requests or authentication dialogs must be completed before the next
step is started.

In the transfer phase, socat watches both streams' read and write file
descriptors via select(), and, when data is available on one side and
can be written to the other side, socat reads it, performs newline
character conversions if required, and writes the data to the write file
descriptor of the other stream, then continues waiting for more data in both
directions.

When one of the streams effectively reaches EOF, the closing phase
begins. Socat transfers the EOF condition to the other stream,
i.e. tries to shutdown only its write stream, thus giving it a chance to
terminate gracefully. For a defined time, socat continues to transfer data in
the other direction, but then closes all remaining channels and terminates.

Socat provides some command line options that modify the behaviour of the
program. They have nothing to do with so called
address options that are used as parts of address specifications.

-V

Print version and available feature information to stdout, and exit.

-?

Print a help text to stdout describing command line options and available address
types, and exit.

-??

Like -?, plus a list of the short names of all available address options. Some options are
platform dependend, so this output is helpful for checking the particular
implementation.

-???

Like -?, plus a list of all available address option names.

-d

Without this option, only fatal and error messages are generated; applying
this option also prints warning messages. See DIAGNOSTICS
for more information.

-d -d

Prints fatal, error, warning, and notice messages.

-d -d -d

Prints fatal, error, warning, notice, and info messages.

-d -d -d -d

Prints fatal, error, warning, notice, info, and debug
messages.

-D

Logs information about file descriptors before starting the transfer phase.

-ly[<facility>]

Writes messages to syslog instead of stderr; severity as defined with -d
option. With optional <facility>, the syslog type can
be selected, default is "daemon".

-lf <logfile>

Writes messages to <logfile> [filename] instead of
stderr.

-ls

Writes messages to stderr (this is the default).

-lp<progname>

Overrides the program name printed in error messages.

-lu

Extends the timestamp of error messages to microsecond resolution. Does not
work when logging to syslog.

-lm[<facility>]

Mixed log mode. During startup messages are printed to stderr; when socat
starts the transfer phase loop or daemon mode (i.e. after opening all
streams and before starting data transfer, or, with listening sockets with
fork option, before the first accept call), it switches logging to syslog.
With optional <facility>, the syslog type can be
selected, default is "daemon".

-v

Writes the transferred data not only to their target streams, but also to
stderr. The output format is text with some conversions for readability, and
prefixed with "> " or "< " indicating flow directions.

-x

Writes the transferred data not only to their target streams, but also to
stderr. The output format is hexadecimal, prefixed with "> " or "< "
indicating flow directions. Can be combined with -v.

-b<size>

Sets the data transfer block <size> [size_t].
At most <size> bytes are transferred per step. Default is 8192 bytes.

-s

By default, socat terminates when an error occurred to prevent the process
from running when some option could not be applied. With this
option, socat is sloppy with errors and tries to continue. Even with this
option, socat will exit on fatals, and will abort connection attempts when
security checks failed.

-t<timeout>

When one channel has reached EOF, the write part of the other channel is shut
down. Then, socat waits <timeout> [timeval] seconds
before terminating. Default is 0.5 seconds.

-u

Uses unidirectional mode. The first address is only used for reading, and the
second address is only used for writing.

-U

Uses unidirectional mode in reverse direction. The first address is only
used for writing, and the second address is only used for reading.

-g

During address option parsing, don't check if the option is considered
useful in the given address environment. Use it if you want to force, e.g.,
appliance of a socket option to a serial device.

With the address command line arguments, the user gives socat instructions and
the necessary information for establishing the byte streams.

An address specification usually consists of an address type
keyword, zero or more required address parameters separated by ':' from the keyword and
from each
other, and zero or more address options separated by ','.

The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some
keywords there exist synonyms ('-' for STDIO, TCP for TCP4). Keywords are case
insensitive.
For a few special address types, the keyword may be omitted:
Address specifications starting with a number are assumed to be FD (raw file
descriptor) addresses;
if a '/' is found before the first ':' or ',', GOPEN (generic file open) is
assumed.

The required number and type of address parameters depend on the address
type. E.g., TCP4 requires a server specification (name or address), and a port
specification (number or service name).

Zero or more address options may be given with each address. They influence the
address in some ways.
Options consist of an option keyword or an option keyword and a value,
separated by '='. Option keywords are case insensitive.
For filtering the options that are useful with an address
type, each option is member of one option group. For
each address type there is a set of option groups allowed. Only options
belonging to one of these address groups may be used (except with option -g).

Address specifications following the above schema are also called single
address specifications.
Two single addresses can be combined with "!!" to form a dual type
address for one channel. Here, the first address is used by socat for reading
data, and the
second address for writing data. There is no way to specify an option only once
for being applied to both single addresses.

Usually, addresses are opened in read/write
mode. When an address is part of a dual address specification, or when
option -u is used, an address might be used
only for reading or for writing. Considering this is important with some
address types.

This section describes the available address types with their keywords,
parameters, and semantics.

CREATE:<filename>

Opens <filename> with creat() and uses the file
descriptor for writing.
This address type requires write-only context, because a file opened with
creat cannot be read from.
<filename> must be a valid existing or not existing path.
If <filename> is a named pipe, creat() might block;
if <filename> refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options:
mode,
user,
group,
unlink-early,
unlink-late,
append
See also: OPEN, GOPEN

EXEC:<command-line>

Forks a sub process that establishes communication with its parent process
and invokes the specified program with execvp().
<command-line> is a simple command
with arguments separated by single spaces. If the program name
contains a '/', the part after the last '/' is taken as ARGV[0]. If the
program name is a relative
path, the execvp() semantics for finding the program via
$PATH
apply. After successful program start, socat writes data to stdin of the
process and reads from its stdout using a UNIX domain socket Generated by
socketpair() per default.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options:
path,
fdin,
fdout,
chroot,
su,
su-d,
nofork,
pty,
stderr,
ctty,
setsid,
pipes,
login,
login,
login
See also: SYSTEM

(Generic open) This address type tries to handle any file system entry
except directories usefully. <filename> may be a
relative or absolute path. If it already exists, its type is checked.
In case of a UNIX domain socket, socat connects; if connecting fails,
socat assumes a datagram socket and uses sendto() calls.
If the entry is not a socket, socat opens it applying the O_APPEND
flag.
If it does not exist, it is opened with flag
O_CREAT as a regular file.
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also:
OPEN,
CREATE,
UNIX-CONNECT

If <filename> already exists, it is opened.
If is does not exist, a named pipe is created and opened.
Note: When a pipe is used for both reading and writing, it works
as echo service.
Note: When a pipe is used for both reading and writing, and socat tries
to write more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat
might block. Consider using option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options:
rdonly,
nonblock,
group,
user,
mode,
unlink-early
See also: unnamed pipe

PIPE

Creates an unnamed pipe and uses it for reading and writing. It works as an
echo, because everything written
to it appeares immediately as read data.
Note: When socat tries to write more bytes than the pipe can queue (Linux
2.4: 2048 bytes), socat might block. Consider, e.g., using
option -b 2048
Option groups: FD
See also: named pipe

PROXY:<proxy>:<hostname>:<port>

Connects to an HTTP proxy server on port 8080 using TCP/IPv4, and sends a CONNECT
request for hostname:port. If the proxy grants access and succeeds to
connect to the target, data transfer between socat and the target can
start. Note that the traffic need not be HTTP but can be an arbitrary
protocol.
Option groups: FD,SOCKET,IP4,TCP,HTTP
Useful options:
proxyport,
ignorecr,
proxyauth,
resolve,
crnl,
bind,
mss,
sourceport,
retry
See also: SOCKS, TCP4

PTY

Generates a pseudo terminal (pty) and uses its master side. Another process
may open the pty's slave side using it like a serial line or terminal.
Option groups: FD,NAMED,PTY,TERMIOS
Useful options:
link,
openpty,
mode,
user,
group
See also:
UNIX-LISTEN,
PIPE,
EXEC, SYSTEM

READLINE

Uses GNU readline and history on stdio to allow editing and reusing input
lines. This requires the GNU readline and
history libraries. Note that stdio should be a (pseudo) terminal device,
otherwise readline does not seem to work.
Option groups: FD,READLINE,TERMIOS
Useful options:
history,
noecho
See also:
STDIO

Forks a sub process that establishes communication with its parent process
and invokes the specified program with system(). Please note that
<shell-command> [string] must
not contain ',' or "!!", and that shell meta characters may have to be
protected.
After successful program start, socat writes data to stdin of the
process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options:
path,
fdin,
fdout,
chroot,
su,
su-d,
nofork,
pty,
stderr,
ctty,
setsid,
pipes,
sigint,
sigquit
See also: EXEC

Connects to <port> [UDP service] on
<host> [IPv4 address] using UDP/IPv4.
Please note that,
due to UDP protocol properties, no real connection is established; data has
to be sent for connecting' to the server, and no end-of-file condition can
be transported.
Option groups: FD,SOCKET,IP4
Useful options:
ttl,
tos,
bind,
sourceport
See also:
UDP4-LISTEN,
TCP4,
UDP6

UDP4-LISTEN:<port>

Waits for a UDP/IPv4 packet arriving on <port>
[UDP service] and connects' back to sender.
Please note that,
due to UDP protocol properties, no real connection is established; data has
to arrive from the peer first, and no end-of-file condition can be
transported. Note that opening
this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4
Useful options:
fork,
bind,
range
See also:
UDP4,
TCP4-LISTEN,
UDP6-LISTEN

UDP6:<host>:<port>

Connects to <port> [UDP service] on
<host> [IPv6 address] using UDP/IPv6. Please
note that, due to UDP protocol properties, no real connection is
established; data has to be sent for connecting' to the server, and no
end-of-file condition can be transported.
Option groups: FD,SOCKET,IP6
Useful options:
ttl,
tos
bind,
sourceport,
See also:
UDP6-LISTEN,
TCP6,
UDP4

UDP6-LISTEN:<port>

Waits for a UDP/IPv6 packet arriving on <port>
[UDP service] and connects' back to sender.
Please note that, due to UDP protocol properties, no
real connection is established; data has to arrive from the peer first, and
no end-of-file condition can be transported. Note that opening
this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6
Useful options:
fork,
bind,
range
See also:
UDP6,
TCP6-LISTEN,
UDP4-LISTEN

UNIX-CONNECT:<filename>

Connects to <filename> assuming it is a UNIX domain
socket.
If <filename> does not exist, this is an error;
if <filename> is not a UNIX domain socket, this is an error;
if <filename> is a UNIX domain socket, but no process is listening, this is
an error.
Option groups: FD,SOCKET,NAMED
See also:
UNIX-LISTEN,
TCP4

UNIX-LISTEN:<filename>

Listens on <filename> using a UNIX domain stream
socket and accepts a connection.
If <filename> exists and is not a socket, this is an error.
If <filename> exists and is a UNIX domain socket, binding to the address
fails (use option unlink-early!).
Note that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD
Useful options:
fork,
umask,
mode,
user,
group,
unlink-early
See also:
UNIX-CONNECT,
TCP4-LISTEN

Address options can be applied to address specifications to influence the
process of opening the addresses and the
properties of the resulting data channels.

For technical reasons not every option can be
applied to every address type; e.g., applying a socket option to a regular file
will fail. To catch most useless combinations as early as in the open phase,
the concept of option groups was introduced. Each option belongs to one
or more option groups. Options can be used only with address types that support
at least one of their option groups (but see option -g).

Address options have data types that their values must conform to.
Every address option consists of just a keyword or a keyword followed by
"=value", where value must conform to the options type.
Some address options manipulate parameters of system calls;
e.g., option sync sets the O_SYNC flag with the open() call.
Other options cause a system or library call; e.g., with option ttl=value'
the setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int)) call is applied.
Other
options set internal socat variables that are used during data transfer;
e.g., crnl' causes explicit character conversions.
A few options have more complex implementations; e.g., su-d
(substuser-delayed) inquires some user and group infos, stores them, and
applies them later after a possible chroot() call.

If multiple options are given to an address, their sequence in the address specification has (almost) no
effect on the sequence of their execution/application. Instead, socat has
built in an option phase model that tries to bring the options in a useful
order. Some options exist in different forms (e.g.,
unlink, unlink-early, unlink-late) to control the time of their execution.

If the same option is specified more than once within one address
specification, with equal or different values, the effect depends on the kind of option. Options
resulting in function calls like setsockopt() cause multiple
invocations. With options that set parameters for a required call like
open()
or set internal flags, the value of the last option occurrence is effective.

The existence or semantics of many options are system dependent. Socat
usually does NOT try to emulate missing libc or kernel features, it just
provides an
interface to the underlying system. So, if an operating system lacks a feature,
the related option is simply not available on this platform.

The following paragraphs introduce just the more common address options. For
a comprehensive reference and to find information about canonical option names,
alias names, option phases, and platforms see file xio.help.

FD option group

This option group contains options that are applied to a UN*X
style file descriptor, no matter how it was generated.
Because all current socat address types are file descriptor based, these
options may be applied to any address.
Note: Some of these options are also member of another option group, that
provides an other, non-fd based mechanism.
For these options, it depends on the actual address type and its option groups
which mechanism is used. The second, non-fd based mechanism is prioritized.

cloexec=<bool>

Sets the FD_CLOEXEC flag with the fcntl() system call to value
<bool>. If set,
the file descriptor is closed on exec() family function calls. Socat
internally handles
this flag for the fds it controls, so in most cases there will be no need to
apply this option.

setlk

Tries to set a discretionary lock to the whole file using the fcntl(fd,
F_SETLK, ...) system call. If the file is already locked, this call results
in an error.

setlkw

Tries to set a discretionary waiting lock to the whole file using the
fcntl(fd, F_SETLKW, ...) system call. If the file is already locked,
this call blocks.

flock-ex

Tries to set a blocking exclusive advisory lock to the file using the
flock(fd, LOCK_EX) system call. Socat hangs in this call if the file
is locked by another process.

flock-ex-nb

Tries to set a nonblocking exclusive advisory lock to the file using the
flock(fd, LOCK_EX) system call. If the file is already locked,
this option results in an error.

flock-sh

Tries to set a blocking shared advisory lock to the file using the
flock(fd, LOCK_SH) system call. Socat hangs in this call if the file
is locked by another process.

flock-sh-nb

Tries to set a nonblocking shared advisory lock to the file using the
flock(fd, LOCK_SH|LOCK_NB) system call. If the file is already locked,
this option results in an error.

lock

Sets a blocking lock on the file. Uses the setlk or flock mechanism
depending on availability on the particular platform. If both are available,
the POSIX variant (setlkw) is selected.

user=<user>

Sets the <user> (owner) of the stream.
If the address is member of the NAMED option group,
socat uses the chown() system call after opening the
file or binding to the UNIX domain socket (race condition!).
Without filesystem entry, socat sets the user of the stream
using the fchown() system call.
These calls might require root privilege.

user-late=<user>

Sets the owner of the fd to <user> with the fchown()
system call after opening
or connecting the channel.
This is useful only on file system entries.

group=<group>

Sets the <group> of the stream.
If the address is member of the NAMED option group,
socat uses the chown() system call after opening the
file or binding to the UNIX domain socket (race condition!).
Without filesystem entry, socat sets the group of the stream
with the fchown() system call.
These calls might require group membership or root privilege.

group-late=<group>

Sets the group of the fd to <group> with the
fchown() system call after opening
or connecting the channel.
This is useful only on file system entries.

mode=<mode>

Sets the <mode> [mode_t] (permissions) of the stream.
If the address is member of the NAMED option group and
uses the open() or creat() call, the mode is applied with these.
If the address is member of the NAMED option group without using these
system calls, socat uses the chmod() system call after opening the
filesystem entry or binding to the UNIX domain socket (race condition!).
Otherwise, socat sets the mode of the stream
using fchmod().
These calls might require ownership or root privilege.

perm-late=<mode>

Sets the permissions of the fd to value <mode>
[mode_t] using the fchmod() system call after
opening or connecting the channel.
This is useful only on file system entries.

append=<bool>

Always writes data to the actual end of file.
If the address is member of the OPEN option group,
socat uses the O_APPEND flag with the open() system call.
Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND) call.

nonblock=<bool>

Tries to open or use file in nonblocking mode. Its only effects are that the
connect() call of TCP addresses does not block, and that opening a
named pipe for reading does not block.
If the address is member of the OPEN option group,
socat uses the O_NONBLOCK flag with the open() system call.
Otherwise, socat applies the fcntl(fd, F_SETFL, O_NONBLOCK) call.

binary

Opens the file in binary mode to avoid implicit line terminator
conversions (Cygwin).

text

Opens the file in text mode to force implicit line terminator conversions
(Cygwin).

noinherit

Does not keep this file open in a spawned process (Cygwin).

NAMED option group

These options work on file system entries.
See also options user, group, and
mode.

user-early=<user>

Changes the <user> (owner) of the file system entry before
accessing it, using the
chown() system call. This call might require root privilege.

group-early=<group>

Changes the <group> of the file system entry before
accessing it, using the
chown() system call. This call might require group membership or root
privilege.

perm-early=<mode>

Changes the <mode> [mode_t] of the file system entry
before accessing it, using the
chmod() system call. This call might require ownership or root
privilege.

umask=<mode>

Sets the umask of the process to <mode> [mode_t] before
accessing the file system entry (useful
with UNIX domain sockets!). This call might affect all further operations
of the socat process!

unlink-early

Unlinks (removes) the file before opening it and even before applying
user-early etc.

unlink

Unlinks (removes) the file before accessing it, but after user-early etc.

unlink-late

Unlinks (removes) the file after opening it to make it inaccessible for
other processes after a short race condition.

OPEN option group

The OPEN group options allow to set flags with the open() system call.
E.g., option creat' sets the O_CREAT flag.
See also options append and
nonblock.

creat=<bool>

Creates the file if it does not exist.

dsync=<bool>

Blocks write() calls until metainfo is physically written to media.

excl=<bool>

With option creat, if file exists this is an error.

largefile=<bool>

On 32 bit systems, allows a file larger than 2^31 bytes.

noctty=<bool>

Does not make this file the controlling terminal.

nofollow=<bool>

Does not follow symbolic links.

nshare=<bool>

Does not allow to share this file with other processes.

rshare=<bool>

Does not allow other processes to open this file for writing.

rsync=<bool>

Blocks write() until metainfo is physically written to media.

sync=<bool>

Blocks write() until data is physically written to media.

rdonly=<bool>

Opens the file for reading only.

wronly=<bool>

Opens the file for writing only.

trunc

Truncates the file to size 0 during opening it.

REG and BLK option group

These options are usually applied to a UN*X file descriptor, but their
semantics make sense only on a file supporting random access.

Applies the ftruncate(fd, <offset>)
(or ftruncate64 if available) system call, thus
truncating the file at the position <offset> [off_t or
off64_t].

PROCESS option group

Options of this group change the process properties instead of just affecting
one data channel.
For EXEC and SYSTEM addresses and for LISTEN and CONNECT type addresses with
option FORK,
these options apply to the child processes instead of the main socat process.

chroot=<directory>

Performs a chroot() operation to <directory>
after processing the address. This call might require root privilege.

chroot-early=<directory>

Performs a chroot() operation to <directory>
before opening the address. This call might require root privilege.

setgid=<group>

Changes the primary <group> of the process after
processing the address. This call might require root privilege.

setgid-early=<group>

Changes the primary <group> of the process before opening
the address. This call might require root privilege.

setuid=<user>

Changes the <user> (owner) of the process after processing
the address. This call might require root privilege.

setuid-early=<user>

Changes the <user> (owner) of the process before opening
the address. This call might require root privilege.

su=<user>

Changes the <user> (owner) and groups of the process after
processing the address. This call might require root privilege.

su-d=<user>

Short name for substuser-delayed.
Changes the <user>
(owner) and groups of the process after processing the address.
The user and his groups are retrieved before a possible
chroot(). This call might require root privilege.

setpgid=<pid_t>

Makes the process a member of the specified process group
<pid_t>. If no value
is given, or if the value is 0 or 1, the process becomes leader of a new
process group.

setsid

Makes the process the leader of a new session.

READLINE option group

These options apply to the readline address type.

history=<filename>

Reads and writes history from/to <filename>.

noprompt

Since version 1.4.0, socat per default tries to determine a prompt -
that is then passed to the readline call - by remembering the last
incomplete line of the output. With this option, socat does not pass a
prompt to the readline call, so it might set the cursor to the first column
of the terminal.

noecho=<pattern>

Specifies a regular pattern for a prompt that prevents the following input
line from being displayed on the screen and from being added to the history.
The prompt is defined as the text that was output to the readline address
after the lastest newline character and before an input character was
typed. The pattern is a regular expression, e.g.
"^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex(7) for details.

prompt=<string>

Passes the string as prompt to the readline function. readline prints this
prompt when stepping through the history. If this string matches a constant
prompt issued by an interactive program on the other socat address,
consistent look and feel can be archieved.

APPLICATION option group

This group contains options that work at data level.
Note that these options only apply to the "raw" data transferred by socat,
but not to protocol data used by addresses like
PROXY.

cr

Converts the default line termination character NL ('\n', 0x0a) to/from CR
('\r', 0x0d) when writing/reading on this channel.

When EOF occurs on this channel, socat ignores it and tries to read more
data (like "tail -f").

SOCKET option group

These options are intended for all kinds of sockets, e.g. IP or UNIX domain. Most are applied with a setsockopt() call.

bind=<sockname>

Binds the socket to the given socket address using the bind() system
call. The form of <sockname> is socket domain dependent:
IP4 and IP6 allow the form [hostname|hostaddress][:(service|port)],
UNIX domain sockets require <filename>.

interface=<interface>

Binds the socket to the given <interface>.
With Linux, this is a string like "eth0".
This option might require root privilege.

Only communicates with directly connected peers, does not use routers.

keepalive

Enables sending keepalives on the socket.

linger=<seconds>

Blocks shutdown() or close() until data transfers have finished
or the given timeout [int] expired.

oobinline

Places out-of-band data in the input data stream.

priority=<priority>

Sets the protocol defined <priority> [<int>] for outgoing
packets.

rcvbuf=<bytes>

Sets the size of the receive buffer after the socket() call to
<bytes> [int]. With TCP
sockets, this value corresponds to the socket's maximal window size.

rcvbuf-late=<bytes>

Sets the size of the receive buffer when the socket is already
connected to <bytes> [int].
With TCP sockets, this value corresponds to the socket's
maximal window size.

rcvlowat=<bytes>

Specifies the minimum number of received bytes [int] until
the socket layer will pass the buffered data to socat.

rcvtimeo=<seconds>

Sets the receive timeout [timeval].

reuseaddr

Allows other sockets to bind to an address even if parts of it (e.g. the
local port) are already in use by socat.

sndbuf=<bytes>

Sets the size of the send buffer after the socket() call to
<bytes> [int].

sndbuf-late=<bytes>

Sets the size of the send buffer when the socket is connected to
<bytes> [int].

sndlowat=<bytes>

Specifies the minimum number of bytes in the send buffer until the socket
layer will send the data to <bytes> [int].

sndtimeo=<seconds>

Sets the send timeout to seconds [timeval].

type=<type>

Sets the type of the socket, usually as argument to the socket() or
socketpair() call, to <type> [int].
Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3
means raw socket.

IP4 and IP6 option groups

These options can be used with IPv4 and IPv6 based sockets.

tos=<tos>

Sets the TOS (type of service) field of outgoing packets to <tos>
[byte] (see RFC 791).

ttl=<ttl>

Sets the TTL (time to live) field of outgoing packets to <ttl>
[byte].

ipoptions=<data>

Sets IP options like source routing. Must be given in binary form,
recommended format is a leading "x" followed by an even number of hex
digits. This option may be used multiple times, data are appended.
E.g., to connect to host 10.0.0.1 via some gateway using a loose source
route, use the gateway as address parameter and set a loose source route
using the option ipoptions=x8307040a000001.
IP options are defined in RFC 791.

mtudiscover=<0|1|2>

Takes 0, 1, 2 to never, want, or always use path MTU discover on this
socket.

TCP option group

These options may be applied to TCP sockets. They work by invoking setsockopt() with the appropriate parameters.

cork

Doesn't send packets smaller than MSS (maximal segment size).

defer-accept

While listening, accepts connections only when data from the peer arrived.

keepcnt=<count>

Sets the number of keepalives before shutting down the socket to
<count> [int].

keepidle=<seconds>

Sets the idle time before sending the first keepalive to <seconds>
[int].

keepintvl=<seconds>

Sets the intervall between two keepalives to <seconds>
[int].

linger2=<seconds>

Sets the time to keep the socket in FIN-WAIT-2 state to <seconds>
[int].

mss=<bytes>

Sets the MSS (maximum segment size) after the socket() call to <bytes>
[int]. This
value is then proposed to the peer with the SYN or SYN/ACK packet.

mss-late=<bytes>

Sets the MSS of the socket after connection has been established to <bytes>
[int].

nodelay

Turns off the Nagle algorithm for measuring the RTT (round trip time).

Sets the maximal number of SYN retransmits during connect to <count>
[int].

UDP and TCP option groups

Here we find options that are related to the network port mechanism and that
thus can be used with UDP and TCP, client and server addresses.

sourceport=<port>

For outgoing (client) TCP and UDP connections, it sets the source
<port> using an extra bind() call.
With TCP or UDP listen addresses, socat immediately shuts down the
connection if the client does not use this sourceport.

lowport

Outgoing (client) TCP and UDP connections with this option use
an unused random source port between 640 and 1023 incl. On UNIX class operating
systems, this requires root privilege, and thus indicates that the
client process is authorized by local root.
TCP and UDP listen addresses with this option immediately shut down the
connection if the client does not use a sourceport <= 1023.
This mechanism can provide limited authorization under some circumstances.

SOCKS option group

When using SOCKS type addresses, some socks specific options can be set.

socksport=<tcp service>

Overrides the default "socks" service or port 1080 for the socks server
port with <TCP service>.

socksuser=<user>

Sends the <user> [string] in the username field to the
socks server. Default is the actual user name ($LOGNAME or $USER).

HTTP option group

Options that can be provided with HTTP type addresses. The only HTTP address
currently implemented is proxy-connect.

proxyport=<TCP service>

Overrides the default HTTP proxy port 8080 with
<TCP service>.

ignorecr

The HTTP protocol requires the use of CR+NL as line terminator. When a proxy
server violates this standard, socat might not understand its answer.
This option directs socat to interprete NL as line terminator and
to ignore CR in the answer. Nevertheless, socat sends CR+NL to the proxy.

proxyauth=<username>:<password>

Provide "basic" authentication to the proxy server. The argument to the
option is used with a "Proxy-Authorization: Base" header in base64 encoded
form.
Note: username and password are visible for every user on the local machine
in the process list; username and password are transferred to the proxy
server unencrypted (base64 encoded) and might be sniffed.

resolve

Per default, socat sends to the proxy a CONNECT request containing the
target hostname. With this option, socat resolves the hostname locally and
sends the IP address.

RANGE option group

These options check if a connecting client is granted access. They can be
applied to listening network sockets.

range=<address-range>

After accepting a connection, tests if the peer is within range. This
option is currently only implemented for IPv4 addresses. Address range has
the form ww.xx.yy.zz/bits, e.g. 10.0.0.0/8. If the client address does not
match, socat issues an error aborting the program
or keeps listening (see option -s).

tcpwrap[=<name>]

Uses Wietse Venema's libwrap (tcpd) library to determine
if the client is allowed to connect. The configuration files are
/etc/hosts.allow and /etc/hosts.deny, see "man 5 hosts_access" for
more information. <name> (type string) is passed to the wrapper functions as daemon
process name. If omitted, the basename of socats invocation (argv[0]) is
passed.
If both tcpwrap and range options are applied to an address, both
conditions must be fulfilled to allow the connection.

LISTEN option group

Options specific to listening sockets.

backlog=<count>

Sets the backlog value passed with the listen() system call to <count>
[int]. Default is 5.

CHILD option group

Options for addresses with multiple connections via child processes.

fork

After establishing a connection, handles its channel in a child process and
keeps the parent process attempting to produce more connections, either by
listening or by connecting in a loop.
SSL-CONNECT and SSL-LISTEN differ in when they actually fork off the child:
SSL-LISTEN forks before the SSL handshake, while SSL-CONNECT forks
afterwards.
RETRY and FOREVER options are not inherited by the child process.

EXEC option group

Options for addresses that invoke a program.

path=<string>

Overrides the PATH environment variable for searching the program with
<string>. This
$PATH value is effective in the child process too.

login

Prefixes argv[0] for the execvp() call with '-', thus making a
shell behave as login shell.

FORK option group

EXEC or SYSTEM addresses invoke a program using a child process and transfer data between socat and the program. The interprocess communication mechanism can be influenced with the following options. Per
default, a socketpair() is created and assigned to stdin and stdout of
the child process, while stderr is inherited from the socat process, and the
child process uses file descriptors 0 and 1 for communicating with the main
socat process.

nofork

Does not fork a subprocess for executing the program, instead calls execvp()
or system() directly from the actual socat instance. This avoids the
overhead of another process between the program and its peer,
but introduces a lot of restrictions:

o

this option can only be applied to the second socat address.

o

the first socat address cannot be OPENSSL or READLINE

o

socat options -b, -t, -D, -l, -v, -x become useless

o

for both addresses, options ignoreeof, cr and crnl become useless

o

for the second address (the one with option nofork), options
append, cloexec, flock, user, group, mode, nonblock,
perm-late, setlk, and setpgid cannot be applied. Some of these could be
used on the first address instead.

pipes

Creates a pair of unnamed pipes for interprocess communication instead of a
socket pair.

openpty

Establishes communication with the sub process using a pseudo terminal
created with openpty() instead of the default (socketpair or ptmx).

ptmx

Establishes communication with the sub process using a pseudo terminal
created by opening /dev/ptmx or /dev/pts instead of the default
(socketpair).

pty

Establishes communication with the sub process using a pseudo terminal
instead of a socket pair. Creates the pty with an available mechanism. If
openpty and ptmx are both available, it uses ptmx because this is POSIX
compliant.

ctty

Makes the pty the controlling tty of the sub process.

stderr

Directs stderr of the sub process to its output channel by making stderr a
dup() of stdout.

fdin=<fdnum>

Assigns the sub processes input channel to its file descriptor
<fdnum>
instead of stdin (0). The program started from the subprocess has to use
this fd for reading data from socat.

fdout=<fdnum>

Assigns the sub processes output channel to its file descriptor
<fdnum>
instead of stdout (1). The program started from the subprocess has to use
this fd for writing data to socat.

sighup, sigint, sigquit

Has socat pass an eventual signal of this type to the sub process.
If no address has this option, socat terminates on these signals.

TERMIOS option group

For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the terminal parameters defined in the UN*X termios mechanism are made available as address option parameters.
Please note that changes of the parameters of your interactive terminal
remain effective after socat's termination, so you might have to enter "reset"
or "stty sane" in your shell afterwards.
For EXEC and SYSTEM addresses with option PTY,
these options apply to the pty by the child processes.

b0

Disconnects the terminal.

b19200

Sets the serial line speed to 19200 baud. Some other rates are possible; use
something like socat -?? |grep ' b[1-9]' to find all speeds supported by
your implementation.
Note: On some operating systems, these options may not be
available. Use ispeed or ospeed
instead.

echo=<bool>

Enables or disables local echo.

icanon=<bool>

Sets or clears canonical mode, enabling line buffering and some special
characters.

Set the baud rate for incoming data on this line.
See also: ospeed, b19200
dif(istrip=<bool>)

iuclc=<bool>

ixany=<bool>

ixoff=<bool>

ixon=<bool>

kill=<byte>

lnext=<byte>

min=<byte>

nl0

Sets the newline delay to 0.

nl1

nldly=<bool>

noflsh=<bool>

ocrnl=<bool>

ofdel=<bool>

ofill=<bool>

olcuc=<bool>

onlcr=<bool>

onlret=<bool>

onocr=<bool>

opost=<bool>

Enables or disables output processing; e.g., converts NL to CR-NL.

ospeed=<unsigned-int>

Set the baud rate for outgoing data on this line.
See also: ispeed, b19200

parenb=<bool>

Enable parity generation on output and parity checking for input.

parmrk=<bool>

parodd=<bool>

pendin=<bool>

quit=<byte>

reprint=<byte>

sane

Brings the terminal to something like a useful default state.

start=<byte>

stop=<byte>

susp=<byte>

swtc=<byte>

tab0

tab1

tab2

tab3

tabdly=<unsigned-int>

time=<byte>

tostop=<bool>

vt0

vt1

vtdly=<bool>

werase=<byte>

xcase=<bool>

xtabs

PTY option group

These options are intended for use with the pty address
type.

link=<filename>

Generates a symbolic link that points to the actual pseudo terminal
(pty). This might help
to solve the problem that ptys are generated with more or less
unpredictable names, making it difficult to directly access the socat
generated pty automatically. With this option, the user can specify a "fix"
point in the file hierarchy that helps him to access the actual pty.

OPENSSL option group

These options apply to the openssl and
openssl-listen address types.

cipher=<cipherlist>

Selects the list of ciphers that may be used for the connection.
See the man page of ciphers, section CIPHER LIST FORMAT, for
detailed information about syntax, values, and default of <cipherlist>.
Several cipher strings may be given, separated by ':'.
Some simple cipher strings:

3DES

Uses a cipher suite with triple DES.

MD5

Uses a cipher suite with MD5.

aNULL

Uses a cipher suite without authentication.

NULL

Does not use encryption.

HIGH

Uses a cipher suite with "high" encryption.
Note that the peer must support the selected property, or the negotiation
will fail.

method=<ssl-method>

Sets the protocol version to be used. Valid strings (not case sensitive)
are:

SSLv2

Select SSL protocol version 2.

SSLv3

Select SSL protocol version 3.

SSLv23

Select SSL protocol version 2 or 3. This is the default when
this option is not provided.

Specifies the file with the certificate and private key for authentication.
The certificate must be in OpenSSL format (*.pem).
With openssl-listen, use of this option is strongly
recommended. Except with cipher aNULL, "no shared ciphers" error will
occur when no certificate is given.

key=<filename>

Specifies the file with the private key. The private key may be in this
file or in the file given with the cert option. The party that has
to proof that it is the owner of a certificate needs the private key.

cafile=<filename>

Specifies the file with the trusted (root) authority certificates. The file
must be in PEM format and should contain one or more certificates. The party
that checks the authentication of its peer trusts only certificates that are
in this file.

capath=<dirname>

Specifies the directory with the trusted (root) certificates. The directory
must contain certificates in PEM format and their hashes (see OpenSSL
documentation)

egd=<filename>

On some systems, openssl requires an explicit source of random data. Specify
the socket name where an entropy gathering daemon like egd provides random
data, e.g. /dev/egd-pool.

pseudo

On systems where openssl cannot find an entropy source and where no entropy
gathering daemon can be utilized, this option activates a mechanism for
providing pseudo entropy. This is archived by taking the current time in
microseconds for feeding the libc pseudo random number generator with an
initial value. openssl is then feeded with output from random() calls.
NOTE:This mechanism is not sufficient for generation of secure keys!

RETRY option group

Options that control retry of some system calls, especially connection
attempts. With version 1.4.0, these options are implemented only for tcp4 and
tcp6 connect and listen addresses and their derivatives socks, proxy, openssl,
and openssl-listen.

retry=<num>

Number of retries before the connection or listen attempt is aborted.
Default is 0, which means just one attempt.

intervall=<timespec>

Time between consecutive attempts (seconds,
[timespec]). Default is 1 second.

If the first character is a decimal digit, the value is read with
strtoul() as unsigned integer specifying a group id. Otherwise, it
must be an existing group name.

int

A number following the rules of the strtol() function with base
"0", i.e. decimal number, octal number with leading "0", or hexadecimal
number with leading "0x". The value must fit into a C int.

interface

A string specifying the device name of a network interface.

IPv4 address

A hostname that is resolved by gethostbyname(), or an
address in numbers-and-dots notation.
Examples: www.dest-unreach.org, dns1, 127.0.0.1

IPv6 address

A hostname that is resolved by getaddrinfo(), getipnodebyname(),
or gethostbyname(), or an
address in hexnumbers-and-colons notation.
Examples: ip6name.domain.org, ::1, 1234:5678:9abc:def0:1234:5678:9abc:def0

long

A number read with strtol(). The value must fit into a C long.

long long

A number read with strtoll(). The value must fit into a C long long.

off_t

An implementation dependend signed number, usually 32 bits, read with strtol
or strtoll.

off64_t

An implementation dependend signed number, usually 64 bits, read with strtol
or strtoll.

Transfers data between STDIO (-) and a
TCP4 connection to port 80 of host
www.domain.org. This example results in an interactive connection similar to
telnet or netcat. The stdin terminal parameters are not changed, so you may
close the relay with ^D or abort it with ^C.

This is similar to the previous example, but you can edit the current line in a
bash like manner (READLINE) and use the
history file .http_history; socat
prints messages about progress (-d -d). The port is specified by service name
(www), and correct network line termination characters (crnl) instead of NL
are used.

socat TCP4-LISTEN:www TCP4:www.domain.org:www

Installs a simple TCP port forwarder. With
TCP4-LISTEN it listens on local port "www" until a
connection comes in, accepts it, then connects to the remote host
(TCP4) and starts data transfer. It will not accept a
second connection.

TCP port forwarder, each side bound to another local IP address
(bind). This example handles an almost
arbitrary number of parallel or consecutive connections by
fork'ing a new
process after each accept(). It provides a little security by
su'ing to user
nobody after forking; it only permits connections from the private 10 network (range);
due to reuseaddr, it allows immediate restart after master process's
termination, even if some child sockets are not completely shut down.
With -lmlocal2, socat logs to stderr until successfully
reaching the accept loop. Further logging is directed to syslog with facility
local2.

A simple server that accepts connections
(TCP4-LISTEN) and fork's a new
child process for each connection; every child acts as single relay.
The client must match the rules for daemon process name "script" in
/etc/hosts.allow and /etc/hosts.deny, otherwise it is refused access (see "man
5 hosts_access").
For EXEC'uting the program, the child process
chroot's
to /home/sandbox, su's to user sandbox, and then starts
the program /home/sandbox/bin/myscript. Socat and
myscript communicate via a pseudo tty (pty); myscript's
stderr is redirected to stdout,
so its error messages are transferred via socat to the connected client.

mail.sh is a shell script, distributed with socat, that implements a
simple
SMTP client. It is programmed to "speak" SMTP on its FDs 3 (in) and 4 (out).
The fdin and fdout options tell socat
to use these FDs for communication with
the program. Because mail.sh inherits stdin and stdout while socat does not
use them, the script can read a
mail body from stdin. Socat makes alias1 your local source address
(bind), cares for correct network line termination
(crnl) and sends
at most 512 data bytes per packet (mss).

socat - /dev/ttyS0,raw,echo=0,crnl

Opens an interactive connection via the serial line, e.g. for talking with a
modem. raw and echo set ttyS0's terminal
parameters to practicable values, crnl
converts to correct newline characters. Consider using
READLINE instead of -'.

With UNIX-LISTEN, socat opens a listening
UNIX domain socket /tmp/.X11-unix/X1. This path corresponds
to local XWindow display :1 on your machine, so XWindow client connections to
DISPLAY=:1 are accepted. Socat then speaks with
the SOCKS4 server host.victim.org that might permit
sourceport 20 based connections due to an FTP related
weakness in its static IP filters. Socat
pretends to be invoked by socksuser nobody, and
requests to be connected to
loopback port 6000 (only weak sockd configurations will allow this). So we get
a connection to the victims XWindow server and, if it does not require MIT
cookies or Kerberos authentication, we can start work. Please note that there
can only be one connection at a time, because TCP can establish only one
session with a given set of addresses and ports.

socat -u /tmp/readdata,seek-end=0,ignoreeof -

This is an example for unidirectional data transfer
(-u). Socat transfers data
from file /tmp/readdata (implicit address GOPEN), starting
at its current end (seek-end=0 lets socat start
reading at current end of file; use seek=0 or no
seek option to first read the existing data) in a "tail -f" like mode
(ignoreeof). The "file"
might also be a listening UNIX domain socket (do not use a seek option then).

EXEC'utes an ssh session to server. Uses a pty for communication between socat and
ssh, makes it ssh's controlling tty (ctty),
and makes this pty the owner of
a new process group (setsid), so ssh accepts the password from socat.

Implements a simple network based message collector.
For each client connecting to port 3334, a new child process is generated (option fork).
All data sent by the clients are append'ed to the file /tmp/in.log.
If the file does not exist, socat creat's it.
Option reuseaddr allows immediate restart of the server
process.

Puts a command line history (READLINE) in front of the EXEC'uted ftp client utility.
This allows editing and reuse of FTP commands for relatively comfortable
browsing through the ftp directory hierarchy. The password is echoed!
pty is required to have ftp issue a prompt.
Nevertheless, there may occur some confusion with the password and FTP
prompts.

On server with modem:

socat TCP4-LISTEN:54321,reuseaddr /dev/ttyS0,nonblock

On client: mkdir $HOME/dev

socat PTY,link=$HOME/dev/vmodem0 TCP4:modemserver.us.org:54321

Installs a TCP4 service on a modemserver and
generates a pseudo terminal
device (PTY) on the client that can be reached under the
symbolic link $HOME/dev/vmodem0. Now
an application on the client that expects a serial line or modem
can be configured to use $HOME/dev/vmodem0; its traffic will be directed
to /dev/ttyS0 on the modem server.

starts a forwarder that accepts connections on port 2022, and directs them
through the proxy daemon listening on port 3128
(proxyport) on host proxy, using the
CONNECT method, where they are authenticated as "user" with "pass" (proxyauth). The proxy
should establish connections to host www.domain.org on port 22 then.

echo |socat -u - file:/tmp/bigfile,create,largefile,seek=100000000000

creates a 100GB sparse file; this requires a file system type that
supports this (ext2, ext3, reiserfs, jfs; not minix, vfat). The operation of
writing 1 byte might take long (reiserfs: some minutes; ext2: "no" time), and
the resulting file can consume some disk space with just its inodes (reiserfs:
2MB; ext2: 16KB).

socat tcp-l:7777,reuseaddr,fork system:'filan -i 0 -s >&2',nofork

listens for incoming TCP connections on port 7777. For each accepted
connection, invokes a shell. This shell has its stdin and stdout directly
connected to the TCP socket (nofork). The shell starts filan and lets it print the socket addresses to
stderr (your terminal window).

echo -e

functions as primitive binary editor: it writes the 4 bytes 000 014 000 000 to
the executable /usr/bin/squid at offset 0x00074420 (this is a real world patch
to make the squid executable from Cygwin run under Windows, actual per May 2004).

Socat uses a logging mechanism that allows to filter messages by severity. The
severities provided are more or less compatible to the appropriate syslog
priority. With one or up to four occurrences of the -d command line option, the
lowest priority of messages that are issued can be selected. Each message
contains a single uppercase character specifying the messages severity (one of
F, E, W, N, I, or D)

FATAL:

Conditions that require unconditional and immediate program termination.

ERROR:

Conditions that prevent proper program processing. Usually the
program is terminated (see option -s).

WARNING:

Something did not function correctly or is in a state where
correct further processing cannot be guaranteed, but might be possible.

NOTICE:

Interesting actions of the program, e.g. for supervising socat in some kind of server mode.

INFO:

Description of what the program does, and maybe why it happens.

DEBUG:

Description of how the program works, all system or library calls and their results.

Log messages can be written to stderr, to a file, or to syslog.

On exit, socat gives status 0 if it terminated due to EOF or inactivity
timeout, with a positive value on error, and with a negative value on fatal
error.